What does everyone use when they're towing their boats to prevent rock chips?

I tow the boat down a gravel road a couple of times year (about 3miles) and I've never had a problem with rocks hitting the boat because I don't go very fast.

The trailer is another story though. It gets really beat up. I've seen some things that hang down off the bumper and something that fits on the trailer as well. Anyone know where to find them or what they're called?